The Juvenile: Stage 2 of a Freshwater Fish's Life

Early in its life, a fish makes the transformation from larva to juvenile; for the first time in its life, it now looks like a miniature version of its own parents. This change is sometimes described as a metamorphosis, although the changes that occur in fish at this time are less dramatic than those seen in caterpillars or tadpoles. Nonetheless, important changes do occur: the fish develops pigmentation and the continuous fin that surrounded the hind part of its body breaks down into distinct dorsal, caudal, anal, and pelvic fins. The body changes shape, from the tiny, glasslike sliver typical of larvae of all species, to a recognizable member of its own.

What drives juvenile fish behavior?

Staying alive is obviously the most important motivation, but as well as dodging predators, juvenile fish need to feed voraciously in order to grow. Each extra millimeter can mean one less predator to worry about Even though foraging exposes juvenile fish to risk staying safe at home is not necessarily an option, because small animals have fewer fat reserves than larger individuals and must feed regularly.

Do different fish use different strategies?

Solving the problem of balancing the benefits of extra feeding against the risk of coming face to face with a predator surprise. Recent research has shown how, in a given situation, some individuals will consistently adopt a “high-risk, high-reward” strategy, while others are far more cautious, taking more of a “safety-first” viewpoint. These different personality types have become known as “bold” and “shy” respectively.

Research on zebra danios (Danio rerio), for instance, has shown that bold individuals will accept something like ten times the level of risk for a particular reward compared to shy fish.The gamble taken by bold fish is that in exposing themselves to this risk they will grow far more rapidly than their shy counterparts, reaching adulthood and the all-important finishing line — reproducing for themselves — sooner. Shy individuals keep out of harm's way and grow slowly, although their ultimate goal is the same. The difference between bold and shy fish can be easily seen in the aquarium; some bolder fish will consistently be first to the food. In addition, some will recover their normal behavior patterns more quickly than others after being introduced to a new tank.

How do juvenile fish stay out of trouble?

Juvenile fish have a number of ways to reduce the risks that go with being small in a dangerous world. One obvious example is in juvenile coloration — younger fish sometimes have different less showy patterns than adults. Juvenile rainbowfish, for example, are often very plain by comparison to the dramatic adult hues, and male guppies do not exhibit their showy color patterns until they reach maturity. Another example is the way in which some fish, such as cichlids, school as youngsters, benefiting from the protection that schooling provides, before leaving the group to establish their own territories when they become adults.